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POZZOLANE AND SLAGS                              147

and are determined by the method already described (see Limestones, Com-
plete Analysis, 7 and 8). By treatment of the insoluble residue with sodium
carbonate (see Limestones, Complete Analysis, 6, b), the silica of the silicates
attacked by the acid is separated. The new residue is disaggregated and
examined by the methods given for clays (q.v., 3-6) the silica and the differ-
ent bases of the silicates not decomposed by hydrochloric acid thus being

Another determination of use in the evaluation of pozzolanic materials
is that of the constituents of the silicates attackable by alkalies by Lunge
and Millberg's method.x Pozzolana and trass contain, as active components,
zeolitic silicates, especially a silicate of aluminium and sodium analogous
to analcite, these being decomposed in the hot by caustic potash. Thus,
by digesting 0-5 gram of the substance with 50 c.c. of 30% caustic potash
solution for about 6 hours on the water-bath, diluting, filtering and deter-
mining the silica and alumina in the nitrate, an indication is obtained of
the technical value of the material. The experiments of the authors men-
tioned above on various specimens of trass and pozzolana show that about
24-28% of silica and 11-13% of alumina pass into solution under this

2.   Technical Examination of Pozzolane

This includes : certain preliminary tests for detecting the presence of
heterogeneous and inert material; the lime absorption test, and especially
tests relating to the fineness, the absolute and apparent density, the setting
and the strength.

1. Presence of Extraneous Matter.—The presence of earthy matters
may be detected by shaking the pozzolana with water and allowing to
settle : pozzolana free from earth deposits rapidly and leaves the liquid
clear. When the pozzolana is heated with caustic potash solution, if earthy
matter is present, the organic substances of the latter colour the potash
solution brown, and the addition of acid then produces a brown precipitate.
Further, on dry distillation, nitrogenous organic matter yields empyreumatic
products, alkaline owing to the presence of ammonia; a pozzolana con-
taining them will give, therefore, ammonia when heated with caustic potash

2. Lime Absorption.—This test yields satisfactory results if a control
test on a good pozzolana of known value is also made. A 10% sugar solu-
tion is left in contact with an excess of spent lime for at least 12 hours,
with frequent shaking. After nitration, the alkalinity of the solution is
determined by means of N-hydrochloric acid, of which I c.c. =0-028
gram CaO.

The test is made by treating 20 grams of pozzolana in a flask with 100
c.c. of the above solution, closing the flask and leaving it for two or three
days, with occasional shaking; the liquid is then filtered through a dry
filter and an aliquot part titrated with N-hydrochloric acid. The quantity
of calcium oxide absorbed by 100 grams of the pozzolana is then calculated.

1 Zeitschr, fur angew. Chem., 1897, p. 428.(q.v., Complete Analysis, 9 and 10).